Floating or anchored production systems have been used to transfer fluids, such as crude oil and natural gas, between subsea installations and the water surface in offshore areas. Whereas, fixed platforms have been common for many years in such production systems, floating platforms have become attractive as petroleum production extends water depths beyond the economic and physical limitations of fixed platforms. Generally, fluid handling lines or conduits are used to transfer the hydrocarbons between the subsea wellheads on the ocean floor and the above-surface platforms. As is known in the art, these lines or conduits commonly are called "production riser pipe strings" which permit the movement of fluids from the wellhead to the platform and "drilling riser pipe strings" which permit drilling pipe strings to be lowered into and raised from the wellhead and provide a conduit through which drilling fluid may be returned to the above-surface installation during drilling operations. In essence, a riser pipe string simply is a plurality of tubular riser pipes connected end-to-end between the wellhead and the drilling installation.
As deeper and deeper hydrocarbon reservoirs are discovered, it seems that ever increasingly complex and sophisticated production systems have been developed, including the riser pipe strings, between the wellhead and the drilling installation. Most riser pipe strings include couplings which fairly rigidly interconnect the riser pipes in an end-to-end string. The couplings of opposing pipe ends interconnect, such as by threaded male and female coupling components. The couplings most often are fabricated at least in part of metal material. Such couplings are quite limiting in resiliency and are compliantly restrained in lateral directions, allowing only limited sway, surge and yaw, all of which is undesirable in subsea applications.
The present invention is directed to providing an extremely simple, yet efficient, riser pipe assembly or system which is very compliant in use and completely eliminates the need for tensile load carrying couplings or connections between the individual pipes of the riser pipe strings. With the present invention, each pipe is supported individually rather than supporting an entire pipe string or substantial portion thereof. In addition, the system of the invention invites the use of low cost, lightweight, corrosion resistant composite riser pipes which, heretofore, have not been popular in wellhead applications.
Heretofore, composite riser pipes have been considered undesirable because of their low value of axial modulus in comparison to higher values of metal riser pipes, such as of steel. The present invention overcomes this limitation by combining the desirable axial modulus of metal in a supporting cable and the desirable features of composite pipes as well.
Still further, prior art systems encounter the problem of individually tensioning each riser pipe in a cluster of riser pipe strings. The present invention solves this problem by avoiding the need for individual tensioning devices. The invention eliminates significant riser length change during use due to temperature, pressure and fluid density changes inside the riser pipes, and also permits a larger diameter, flexible riser.